The present invention relates to a wafer inspecting apparatus, and particularly to a wafer inspecting apparatus for inspecting crystal defects such as precipitates or stacking faults in a silicon wafer in such a manner as to distinguish the crystal defects from foreign matters adhering on the surface of the silicon wafer.
As the level of integration of LSIs (Large Scale Integrated Circuits) has been enhanced, there has arisen a large problem in terms of reduction in percent non-defective and reliability resulting from failures of MOS (Metal Oxide Semiconductor) transistors constituting main parts of the LSIs. The failures of MOS transistors are typically caused by dielectric breakdown of gate oxide films and excess leakage currents at junctions. That is to say, most of the failures of MOS transistors directly or indirectly result from crystal defects in silicon substrates. To be more specific, in an LSI fabrication process, if a crystal defect exists in a surface region, to be oxidized into a silicon oxide film, of a silicon substrate, then a structural defect is formed in the silicon oxide film, causing dielectric breakdown upon operation of the LSI; while if a crystal defect exists in a depletion layer of a junction, there occurs a large amount of a leakage current therein. In this way, the presence of a crystal defect in a surface region, in which elements are formed, of a silicon substrate is undesirable because such a crystal defect causes a failure of a MOS transistor. For this reason, measurement of these crystal defects is important in quality control of silicon crystals. In this case, it may be desirable that crystal defects in a wafer be measured in such a manner as to be distinguished from foreign matters on the surface of the wafer, because the countermeasure taken against crystal defects in the wafer is different from that taken against foreign matters on the surface of the wafer.
One method has been disclosed in Japanese Patent Laid-open No. Hei 7-318500. In this method, infrared rays are rendered incident on a silicon substrate in the direction perpendicular thereto, and scattered rays are detected at the Brewster angle of silicon with respect to an axis perpendicular to the surface for each polarized component, to detect internal defects and surface foreign matters in such a manner as to make a distinction therebetween. The principle of this method is based on the fact that in regard to the transmittance of scattered rays which are scattered from an internal defect to the surface of a silicon substrate, the dependence of the intensity of a detection signal of the scattered rays on the polarization direction becomes largest when the scattered rays are detected at the Brewster angle of silicon.
Another method has been reported by Moriya and others in Proceedings of the 44-th Joint Meeting on Applied Physics, No. 1, p. 312, 1997, in which surface foreign matters and internal defects are detected using obliquely incident irradiation rays. In this method, a scattering substance detected by irradiation of S-polarized rays is taken as a surface foreign matter, and a scattering substance detected by irradiation of P-polarized rays is taken as an internal defect.
A further method has been reported by Kurihara in Journal of Electronic Material, PP. 50 to 56, February, 1997. In this method, the surface of a wafer is irradiated with rays in the direction perpendicular thereto, and foreign matters and irregular defects on the surface of the wafer are detected in such a manner as to make a distinction therebetween on the basis of a difference in angle distribution of the scattered rays.
In addition, Japanese Patent Laid-open No. Hei 6-345662 has disclosed a method in which the surface to be detected is irradiated with two kinds of laser rays having different wavelengths, and the presence or absence of defects on the surface is determined on the basis of a correlation between signals of the different scattered rays having the different wavelengths and scattered from the defects.
The above-described methods in which it is determined, by making use of polarization of scattered rays, whether a detected scattering substance is a surface foreign matter or an internal defect has the following problems:
The first problem is that when an object to be detected is irradiated with S-polarized rays, the incident rays interfere with reflected rays from the surface of the object and thereby the reflected rays are shifted from the incident rays by a phase of 180.degree., to cause a phenomenon that the surface becomes dark. On the contrary, when an object to be detected is irradiated with P-polarized rays, such a phenomenon occurs little. Taking the phenomenon into account, when a foreign matter having a particle size larger than the wavelength of irradiation rays is irradiated with the irradiation rays, the intensity of scattered rays from the foreign matter in the case using S-polarized rays as the irradiation rays is larger than the intensity of scattered rays from the foreign matter in the case of using P-polarized rays as the irradiation rays. On the contrary, for a foreign matter having a particle size sufficiently smaller than the wavelength of irradiation rays, the intensity of scattered rays from the foreign matter in the case of using P-polarized rays as the irradiation rays is larger than the intensity of scattered rays from the foreign matter in the case of using S-polarized rays as the irradiation rays, because of the interference effect. Accordingly, even if the intensity of scattered rays from a scattering substance in the case of using P-polarized irradiation rays or the intensity of scattered rays of a P-polarized component from the scattering substance is larger than the intensity of scattered rays from the scattering substance in the case of using S-polarized irradiation rays or the intensity of scattered rays of an S-polarized component from the scattering substance, it cannot be determined that the scattering substance is an internal defect.
The second problem is that the polarization state of scattered rays from a scattering substance is changed depending on whether the material of the scattering substance is anisotropic or isotropic. For example, in the case where irradiation rays are scattered from a scattering substance having a particle size sufficiently smaller than the wavelength of the irradiation rays (measurement of micro-defects are generally equivalent to the case), it is apparent from the Rayleigh scattering theory that the polarization direction of the scattered rays from the scattering substance is the same as the irradiation direction of the irradiation rays insofar as the material of the scattering substance is isotropic. Accordingly, in the case where the scattering substance is an isotropic internal defect, there occur only the scattered rays which are scattered in the same polarization direction as that of the irradiation rays. However, in the case where the scattering substance is an anisotropic internal defect, the polarization direction of the scattered rays from the scattering substance is changed depending on the degree of the anisotropy of the scattering substance.
In view of the foregoing, it is undesirable to generally adopt a method of distinguishing a surface foreign matter and an internal defect from each other by making use of polarization information of irradiation rays or scattered rays.
Next, the above-described method in which the surface of a wafer is irradiated with irradiation rays in the direction perpendicular thereto has a problem. In the vertical incidence of irradiation rays, all polarized components are parallel to the surface irrespective of the polarization direction of the irradiation rays, so that the irradiation rays interfere with reflected rays irrespective of the polarization direction. Such interference between the irradiation rays and reflected rays makes small the intensity of the irradiation rays on the surface. This makes it difficult to detect a micro-defect.